1. Field of the Invention
The present invention relates to a radiation imaging apparatus for performing radiation imaging using a radiopaque dye, and a method of controlling the same.
2. Description of the Related Art
Angiography by an X-ray diagnostic imaging apparatus is done using a radiopaque dye. When blood vessels in which a radiopaque dye is injected are radiographed by X-ray radiation imaging, an X-ray absorption difference is generated between the blood vessels and other tissues, and the blood vessel form or blood flow state can be visualized more clearly. This allows the doctor to make a more correct diagnosis or treatment.
A common method of angiography using a radiopaque dye is DSA (Digital Subtraction Angiography). In DSA, an object is radiographed by X-ray radiation imaging to obtain an image before radiopaque dye injection. This image is called a mask image. Next, a radiopaque dye is injected into blood vessels. The object is radiographed by X-ray imaging to obtain an image after radiopaque dye injection. This image is called a contrast image. The difference between the mask image and the contrast image is calculated, and the background including bones and organs is erased, thereby obtaining an angiographic image containing only a blood vessel image.
To generate an angiographic image containing only blood vessels, continuous images are obtained by radiographing the movement of a radiopaque dye over a plurality of frames, and the peak value is obtained at each pixel position. Based on the peak value of each pixel, images with the highest radiopaque dye density are composited to generate a blood vessel angiographic image.
The irradiation for X-ray imaging is performed at a preset fixed frame rate. Generally, a technician sets the frame rate in accordance with the body part to be radiographed. There also exists an apparatus which allows a technician to manually change the frame rate during X-ray imaging.
A technique of determining the frame rate of X-ray imaging is disclosed in Japanese Patent Laid-Open No. 5-192319 (to be referred to as patent reference 1 hereinafter). Patent reference 1 proposes an X-ray diagnostic apparatus which controls X-ray irradiation in accordance with a change in an object. This apparatus raises the frame rate upon determining that a radiopaque dye enters the irradiation field.
However, the prior art has the following problems.
The amount of use of the radiopaque dye is preferably small in order to suppress the side effect. However, when the amount of use is small, it is necessary to perform imaging at a high frame rate without failing to radiograph images. This leads to excess imaging and an increase in the irradiation dose at a portion with a slow blood flow. If imaging is done at a low frame rate to prevent excess irradiation, an imaging failure occurs at a portion with a fast blood flow, and the obtained angiographic image contains breaks in the blood vessels. That is, excess or deficient irradiation occurs depending on the moving speed of the radiopaque dye.
To manually deal with a change in the blood flow velocity, the technician raises the frame rate when the blood flow velocity increases and lowers the frame rate when the blood flow velocity decreases by operating a user interface (UI) while visually observing, on a monitor, the blood vessel image that is being radiographed. In this method, the reaction speed of human eye (human sensation) affects the frame rate adjustment. It is therefore difficult to avoid excess imaging or imaging failure.
In patent reference 1, it is possible to suppress the total irradiation dose of a patient by limiting the high frame rate period to, for example, a period when the radiopaque dye is present in the irradiation field. When determining that the radiopaque dye enters the irradiation field, imaging using a high frame rate is performed. This prevents an imaging failure but cannot solve the problem of the high irradiation dose caused by excess imaging at a portion with a slow blood flow.